Method and materials for disposing of radioactive waste



tates ate United This invention relates to a method for disposing of radioactive waste and particularly to isolating and disposing of radioactive wastes contained in waters or vapors.

The persistence of radioactivity in radioactive waste materials and its propensity for dangerously contaminating surrounding substances causes serious problems in disposing of these waste materials. Extremely small quantities of radioactive material may dangerously contaminate large bodies of water and such material buried in the ground finds its way into crops and subterranean water. Even the oceans are not sufliciently large reservoirs for these materials unless they are permanently and safely isolated and contained before disposing of them.

It is an object of this invention to provide a method of isolating radioactive material contained in water or gas.

It is another object of this invention to provide an improved method for permanently containing radioactive waste to prevent it from dissipating into surrounding material.

Briefly, the present invention comprises impregnating radioactive waste material into granules or pellets of absorbent ceramic material which are constituted to be vitrifiable or fusible and thereby capable of producing a surface around the exterior of each granule that is impervious to agents capable of leaching out the radioactive material captured within. The absorbent ceramic material comprises two major components; an expanded mineral absorbent, and a fusible ceramic bonding material or agent that is capable of holding the finely divided expanded mineral particles to each other and which forms therewith a matrix or network which is capable of being fused, glazed, or vitrified to seal its surfaces in a manner to prevent leaching out of any radioactive materials absorbed within the interstices of the matrix. The absorbent ceramic material may contain other components to increase its utility as will be described hereinafter.

The method of this invention includes the removal of and segregation of radioactive material that is dissolved, carried, or entrained in water by utilizing one or more of the processes of absorption, adsorption and capillary condensation which operate to carry the radioactive materials into the pores or interstices of the ceramic granules. When the pores are filled with radioactive material and liquid carrier, the granules are dried whereby the liquid is removed as vapor while the radioactive material remains impregnated in the interior of the granules. The sorption and drying processes may be repeated until the absorbent ceramic granules are substantially incapable of holding more radioactive waste. When the granules are saturated with radioactive waste they are subjected to sufiicient heat to glaze or fuse their exterior surfaces, in particular, into a continuous vitreous film which, upon cooling, forms an exterior layer that prevents the leaching out of the radioactive material trapped within the granules.

The absorbent ceramic material is preferably in the form of small discrete granules or particles, having a major dimension of from about inch to about /2 inch; however smaller or larger particles may be employed. The expanded mineral of the particles may be expanded vermiculite, obsidian, pitchstone, pumice, silica,

perlite, or mixtures of these or other suitable porous, ex panded materials normally containing small percentages of entrapped water in their nonexpanded state. The mineral component in an expanded condition is preferably fractured or ground or otherwise reduced to fine particle size, such as a powder, so as to be uniformly distributed throughout the ultimate granule.

The fusible bonding component may be a swelling type of bentonite, kaolin, or other suitable clays or mixtures of these.

It is an embodiment of this invention to also include a desiccant component in the absorbent ceramic material which permits adsorption followed by capillary condensation within the pores of the granules and thereby entraps radioactive material entrained in a vapor. Solid desiccants such as calcium sulphate, calcium oxide, magnesium oxide, barium oxide, calcium chloride, lithium chloride, sodium hydroxide, phosphorous pentoxide, phosphoric acid, etc., may be employed.

The criteria for determining the composition of the granules are that sufficient fusible bonding material must be present to bond the expanded mineral into firm granules. The fusible bonding material may constitute from about 5% to about 50% by weight of the granules and preferably constitutes from about 10% to about 20% by weight.

The amount of expanded cellular mineral in each granule will determine the liquid capacity of the granules and the capacity to hold radioactive materials. It is therefore desirable to have a relatively high concentration of expanded mineral; however, from about 50% to about 95% by weight of absorbent may be employed. Preferably to by weight of each granule will be expanded cellular mineral.

The amount of desiccant when employed should be related to the liquid holding capacity of the granules and should be enough to permit sufficient adsorption commensurate with the magnitude of available capillary condensation. The desiccant may constitute from 0% to about 30% by weight of the granules, and preferably, when used, constitute from about 10% to about 25% by weight.

A preferred method of making the absorbent ceramic material consists of intimately mixing the expanded cellular mineral component and the fusible bonding material and adding water in suflicient quantity to form a paste. The paste is extruded through a suitable die and the extrudate is chopped into pellets of uniform length. The pellets are then air dried at a temperature of between C. and about 300 C. to remove all liquid phase water after which they are calcined at a temperature of from about 800 C. to about 900 C. to form firm granules by establishing the bond and removing water of hydration.

The absorbent ceramic material may also be formed by blending the mixed powder with stearates or other oxidizable bonding materials and pelleting the mixture by compressing the powder into small pellets, for example of cylindrical form. Pellets thus constructed are calcined to remove the organic binder and to bond the particles together. The granules of ceramic absorbent made by either method and thus constituted are extremely porous and will absorb by capillary action 1.25 ml. of solution or more per gram of absorbent ceramic material.

l he desiccant when used is preferably incorporated into the formed granules by absorbing a solution of desiccant, eg a solution of (32101 LiOl, etc. into the pores of the granules and then evaporating the liquid water and heating the resultant impregnated granules to a sufiiciently high temperature to drive off water of hydration, e.g. 100 C. to 300 C. The concentration of the impregnat- 3 ing solution will determine the ultimate proportion of desiccant in the finished granules.

The process of this invention may be efiected in various ways; however it is presently preferred to place a bed of absorbent ceramic granules in a closed vessel and to introduce only enough contaminated water into the vessel to saturate the pores of the granules. A stream of hot air at a temperature of from about 100 C. to about 300 C. is then passed through the bed of saturated granules to evaporate liquid water from their interstices. When all of the water is removed the pellets in the vessel are dry and impregnated with the radioactive waste material that was formerly carried by the water. Experience with the process will provide an operator with knowledge of the capacity of a specific ceramic material that he is using for absorbing radioactive material.

When the absorbent granules are saturated with radioactive material and air dried they may, either in the same vessel or in a different container, be subjected to temperatures in the range of 1200 C. or higher for a sufiicient period of time to eifect fusion or glazing of the exterior surface of each pellet. Each pellet is sufficiently glazed to form a continuous coating around its entire exterior surface thereby sealing the radioactive waste material within the pores of the pellet and permanently containing it to prevent it from being leached out, into or by, a surrounding medium. The pellets thus glazed and sealed may be cast into concrete or encased in a sealed metal or concrete container after which they may be buried safely in the ground or submerged in the ocean.

The process for removing radioactive Waste particles from moist air or other gas is similar tothat just described. The pellets which are constituted as hereinbefore described to contain a dry desiccant are placed in a vessel as a bed and the Water vapor in the moist contaminated gas stream is adsorbed followed by capillary condensation within the pores of the pellets wherein it entraps radioactive waste material. When the pores are filled with water and radioactive waste, the contaminated gas stream is stopped and drying with hot air to evaporate liquid water is effected until all water is removed and the radioactive material remains deposited and impregnated in the pellets. It may be desirable to recirculate. the contaminated gas stream or pass it through two or more beds of granules in the same or different vessels until the effluent gas from the process shows adequate removal of the radioactive material from the stream. The pellets, after being impregnated and dried, are sealed by fusion in the same manner as hereinbefore described.

In removing radioactive waste material from gases or vapors, it is a further embodiment of this invention to introduce steam into the contaminated gas stream before passing it into contact with the adsorbing ceramic granules. The humidity of the contaminated gas stream may thus be controlled to insure that suflicient water will be present to permit the processes of adsorption and capil lary condensation to deposit the radioactive material within the pores of the granules. When multi-stage removal of radioactive waste is effected, that is when the contaminated gas is passed serially through a plurality of beds of granules or when it is recycled, it is particularly desirable to maintain high humidity by introducing steam into the contaminated gas between stages. A relative humidity of from about 50% to about is preferably maintained in the contaminated gas.

Having thus described the invention, what is claimed is:

1. The method of isolating radioactive waste material entrained in a gas which comprises introducing steam into said gas, passing said gas through a bed of discrete granules of absorbent ceramic material characterized by being constituted of finely subdivided particles of porous mineral absorbent and fusible ceramic material, said granules being impregnated with a solid desiccant whereby the water vapor in the gas is adsorbed followed by capillary condensation to entrain said radioactive material in the interstices of said granules, evaporating wvater from said interstices and heating said granules to fuse said ceramic material enough to seal the interstices at the surface of said discrete granules thereby entrapping the radioactive material therein.

2. The rnethod of isolating radioactive waste material entrained in a humid gas which comprises passing said gas through a bed of discrete granules of absorbent ceramic material characterized by being constituted of finely subdivided particles of porous miner-a1 absorbent and fusible ceramic material, said granules being impregnated with a solid desiccant whereby the water vapor in the gas is adsorbed followed by capillary condensation to entrain said radioactive material in the interstices of said granules, introducing steam into the gas flowing from said bed, passing the resultant humidified gas into a second bed of said granules, whereby the water vapor is adsorbed followed by capillary condensation to entrain radioactive material in the interstices of the granules in said second bed, evaporating water from said interstices and heating said granules to fuse said ceramic material enough to seal the interstices at the outer surface of each of said granules in both beds thereof.

3. The method set forth in claim 1, further characterized in that steam is introduced into said gas in sufficient quantity to raise the relative humidity of said gas to from about 50% to about 100%.

References Cited in the file of this patent UNITED STATES PATENTS 2,616,847 Ginell Nov. 4, 1952 2,918,700 Hatch Dec. 29, 1959 2,918,717 Struxness Dec. 29, 1959 2,961,399 Alberti Nov. 22, 1960 

1. THE METHOD OF ISOLATING RADIOACTIVE WASTE MATERIAL ENTRAINED IN A GAS WHICH COMPRISES INTRODUCING STEAM INTO SAID GAS, PASSING SAID GAS THROUGH A BED OF DISCRETE GRANULES OF ABSORBENT CERAMIC MATERIAL CHARACTERIZED BY BEING CONSTITUTED OF FINELY SUBDIVIDED PARTICLES OF POROUS MINERAL ABSORBENT AND FUSIBLE CERAMIC MATERIAL, SAID GRANULES BEING IMPREGNATED WITH A SOLID DESICCANT WHEREBY THE WATER VAPOR IN THE GAS IS ABSORBED FOLLOWED BY CAPILLARY CONDENSATION TO ENTRAIN SAID RADIOACTIVE MATERIAL IN THE INTERSTICES OF SAID GRANULES, EVAPORATING WATER FROM SAID INTERSTICES AND HEATING SAID GRANULES TO FUSE SAID CERAMIC MATERIAL ENOUGH TO SEAL THE INTERSTICES AT THE SURFACE OF SAID DISCRETE GRANULES THEREBY ENTRAPPING THE RADIOACTIVE MATERIAL THEREIN. 